1. Field of the Invention
The present invention relates to the performance of high average power solid-state lasers, and more specifically, it relates to techniques for optically pumping thin solid-state laser media.
2. Description of Related Art
The performance of high average power solid-state lasers is in large part determined by the geometry of pump light delivery to the gain medium and on the intensive cooling that must accompany it. For example, some devices utilize pump light impinging on a face of a slab or along the barrel of a rod in a geometry commonly referred to as side pumping. In some cases, it is desirable to utilize an end-pumping geometry in order to achieve high delivery efficiency. An example of such a case arises when the absorption length of the laser material is limited. Side pumping and end pumping schemes rely on side cooling. Side cooling induces stress and optical deformations that limit the quality and ultimately the level of output power possible by either end or side pumping schemes.
Thin disk or active mirror configurations have recently been demonstrated at continuous wave output powers exceeding 1 kW with the promise of very high beam quality. Today, the dominant development of this type of laser is being pursued at the University of Stuttgart in a group headed by Prof. Adolf Geissen. See A. Giesen, H. Hugel, A. Voss, K. Wittig, U. Brauch, H. Opower, xe2x80x9cScalable Concept for Diode-Pumped High-Power solid-State Lasersxe2x80x9d, App. Phys. B, 58, 365-372 (1994). The fundamental idea of the approach, which enables the generation of high average power with high beam quality, is illustrated in FIG. 1 where it is seen that the thermal gradients 10 in the laser disk 12 are arranged to be parallel to the laser radiation 14 propagation direction from disk laser 12. In the figure, the thermal gradient 10 is arranged in this manner to avoid phase aberrations on the laser beam 14.
By keeping the thermal gradients parallel to the beam propagation direction, to first order, thermal aberrations cannot impart a phase aberration on the laser beam. To understand the average power scaling potential of this approach, one then needs only to consider the maximum thermal power that can be generated in the laser crystal without fracture. In a first approximation, the maximum laser output power that can be generated is proportional to the maximum thermal power dissipation that can be handled by the laser crystal. The laser output power per unit area Plaser is proportional to the fracture strength of a thin disk, which varies inversely as the thickness of the disk, which leads to the following scaling law,       P    laser    ∝      1    t  
where t is the thickness of the thin disk. So, to maximize the average power capability, one is driven to use very thin disks. This in turn drives one to develop multipass pump geometries because the use of thin disks implies very short pump absorption distances. In the very thin gain samples necessary to avoid fracture under intense pumping, the pump beam is re-imaged through the sample up to 16 times in the Stuttgart approach to increase the net absorption path. The very complicated pump geometry (FIG. 2) that this necessitates, and the obvious limitations to high average power scaling that it imposes, are issues that the present thin disk concept recognizes and addresses. FIG. 2 shows the thin disk geometry developed by the Stuttgart group in which the pump radiation 20 from a pump fiber bundle 22 is multipassed through the thin disk sample 24 to increase absorption.
The growth of ASE is another feature of the presently pursued Stuttgart thin disk technology that limits its average power scaling potential. As shown in FIG. 3, because the thin disk 30 laser gain has to be sufficient to support efficient lasing in its thin direction, the transverse size of the gain region is limited due to the onset of parasitic lasing 32. The main reason for this sensitivity to ASE in the presently pursued Stuttgart thin disk approach is the large solid angle that is confined by total internal reflection (TIR) at the top surface 34 of the gain element. Due to TIR at the top surface, very long path rays can be confined within the gain loaded sample and effectively compete against the extracting laser beam for the gain media""s stored energy. The present composite thin disk concept recognizes and addresses this issue. FIG. 3 illustrates the sensitivity of the thin disk architecture being pursued by the Stuttgart group to ASE and parasitic lasing because of the large solid angle confined within the gain medium at the Yb:YAG-air interface.
It is an object of the present invention to provide methods and apparatuses by which the pump absorption length limitation can be circumvented.
It is another object to optically pump a thin sheet of laser material with high delivery efficiency while exposing one of the large faces of the gain medium for cooling.
These and other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.
The invention relies on bonding of a thin disk of laser material to a similar disk of a material closely matched in refractive index so that pump light introduced through edges of the composite structure is trapped and passes through the gain medium repeatedly. The approach is better described as edge pumping with face cooling. High absorption is possible using moderate concentrations of dopant while minimizing the laser medium thickness.
A key aspect of this invention is the efficient delivery of pump light to a thin-disk laser medium of minimum thickness. This maximizes the laser output because in face-cooled disk geometry, for a prescribed maximum peak surface stress (that avoids fracture), the thinner the disk the higher the heat flux it will manage.
An optical advantage is also gained from the xe2x80x9cthinnessxe2x80x9d of the laser gain medium in that the fraction of spontaneous emission that remains within the solid angle of the gain medium is proportional to the square of the thickness of the gain medium. Amplified spontaneous emission (ASE) limits the transverse dimensions of an aperture in complex ways related to the geometric details surrounding the gain medium. However, in general, if the detailed geometry is designed properly, a larger transverse aperture is possible with thinner laser medium dimensions.
The present invention utilizes all of the embodiments of U.S. patent application Ser. No. 09/237,142, titled xe2x80x9cHigh Average Power Scaleable Thin-Disk Laserxe2x80x9d by Beach et al., filed Jan. 25, 1999 and incorporated herein by reference. Embodiments of lens duct designs usable in the present invention are described in U.S. Pat. No. 5,307,430, which is incorporated herein by reference.